The present invention relates to a direct-current motor, and more particularly to an apparatus for removing torque ripples in a direct-current motor.
Torque ripples, which are indicative of periodic variations in the output torque of a direct-current motor while the latter makes one revolution, become larger the smaller the number of phases of armature coils is. Direct-current motors which are normally found in audio equipment are mostly of two phases or three phases, and hence undergo greater output torque variations or torque ripples.
Where such motors are used on cassette tape recorders, turntables, or videotape recorders, reproduced sounds tend to be distorted to a large degree as these devices are quite susceptible to wow and flutter. Use of flywheels to eliminate distortions in sound reproduction fails to achieve complete removal of speed variations. The flywheels render the overall apparatus relatively large and heavy.
Various means for controlling direct-current motors to rotate at constant speeds are known. One such means comprises a shaft encoder for improving the responsiveness of an output with respect to speed variations. With the shaft encoder incorporated, however, improved sound reproduction cannot be accomplished because of limitations on removal of speed variations due to torque ripples. Furthermore, an increased number of manufacturing steps are required to equip direct-current motors with shaft encoders, resulting in a poor rate of mass production. Another constant-speed control means comprises an electrical circuit which is less costly and needs no additional mechanism. Such an electrical circuit is generally in the form of a bridge servo circuit which is widely used to remove wow and flutter. The electrical control means, however, suffers two major difficulties. First, improved constant-speed performance requires fine adjustment of various constants of the circuit, with a resulting increase in the number of manufacturing steps. Secondly, torque ripples remain unremoved as they are, and serve as a source of variations in the speed of rotation of a capstan and hence wow and flutter.
Where the take-up reel of a cassette deck is driven by a direct-current motor during the mode of sound reproduction, the following two requirements must be met: The first requirement is that a magnetic tape to be wound up be kept under constant tension. If the tension that the tape is subjected to varied, the amount of slippage between a capstan and a pinch roller would change, so that the speed of travel of the tape would undergo variations. With conventional systems, the drive motor is supplied with a constant current only, and hence produces a constant torque. Since the diameter of the coiled magnetic tape is larger when the magnetic tape is fully wound up than when the magnetic tape starts being wound, the magnetic tape is tensioned to varying degrees while the tape is being wound on the take-up reel, with the result that the speed of travel of the magnetic tape will vary. The second requirement is removal of torque ripples. The direct-current motor for driving the take-up reel has armature coils of two or three phases, which contribute to generation of torque ripples that amount to 20%.about.30% of the drive torque. Such large torque ripples cause the magnetic tape to run at different speeds, which are a primary cause of wow and flutter.
In order to reduce wow and flutter to a range of 0.03% or less in WRMS, it is necessary to lower torque ripples down to about 1%, which has been found by actual measurements. Since it is difficult to gain such a value of torque ripples, a friction mechanism such as a felt member is generally used to remove torque ripples. This mechanism is however disadvantageous in that its ripple filtering action is unstable and the mechanism fails to work when it gets worn out. In addition, the friction machanism should be accompanied by another mechanism for disengaging the felt member during the fast-forward mode to keep the felt member from being adversely affected by large power transmission from the source of drive or the motor.
When direct-current motors are used as servomotors, servo characteristics are rendered uneven by variations in the output torque.
With a view to removing torque ripples and uniformizing a combined torque, it has been proposed as disclosed in Japanese Unexamined Patent Publication No. 54-68914 to magnetize a permanent magnet rotor in the pattern of a sine wave and to supply a stator armature coil with a sinusoidal current through a position sensor for producing a sinusoidal output indicative of an angular position of the rotor. According to the proposed technique, it is quite difficult to magnetize the rotor in the pattern of a sine wave.